Newly discovered sulfur-containing scent molecules in sweat produced by a bacterial enzyme point to our smelly past and could lead to an odor-free future.
The modern human nose is somewhat insensitive to the subtle fragrances emitted by the axillary region of the species and generally considers any odor as unpleasant. However, the Western blanket ban on body odor belies an aromatic complexity that can provide important clues about chemical communication between our ancestors and the delicate undertones wafting between even our modern selves that try to hide with a whole industry of deodorants and antiperspirants.
Indeed, the human armpit contains a cocktail of substances including various androstenone derivatives and volatile fatty acids, as well as sulfur-containing compounds about which little was known until recently. Now, two Swiss research teams have identified several of the main sulfur-containing components, as well as a bacterial enzyme that seems to be responsible for the release of these compounds.
The R and S forms of certain thiols are found in axillary sweat and have distinct pungent odors (Image by David Bradley)
Anthony Clark of perfumes and flavorings company Firmenich SA enlisted 30 volunteers to work up a heavy sweat on exercise machines and in the sauna. His team then collected sweat from the volunteer armpits and sterilized it. Armpit sweat starts out odourless, but the diverse microbial flora and fauna living there, including strains of Corynebacteria and Staphylococcus, transform the components of sweat into smelly substances. They then tested the effects of different bacterial strains on the volunteers’ sweat. “Of the microorganisms we tested, S. haemolyticus produced the most sulfurous scent,” reports Clark. Isolation and analysis of these decomposition products by gas chromatography, mass spectrometry, and the trained noses of perfumers revealed the presence of at least eight sulfanyl alcohols – compounds containing both OH and SH groups.
One particular compound, identified as 3-methyl-3-sulfanylhexan-1-ol, is present in armpit sweat exposed to S. haemolyticus at a concentration of 4 parts per billion. The left-handed, S, enantiomer of this compound smells of sweat and onion, while the mirror-image molecule, the right-handed R-form, has a fruity, grapefruit-like odor. “Sweat broken down by S. haemolyticus contained the (S)- and (R)-forms in a 3:1 ratio,” adds Clark.
Andreas Natsch and his colleagues at another flavors and fragrances company, Givaudan Schweiz AG, also identified three additional sulfanyl alcohols of similar structure and equally pungent odor. They reported 3-methyl-3-sulfanylhexan-1-ol as a component of underarm sweat. Givaudan has even patented one of the substances as a potential artificial flavoring with a meaty onion-like taste. They also detected another compound that is known to be present in passion fruit and cabernet wines.
Despite our best efforts to mask these compounds, they produce intense odors even at extremely low concentrations in sweat. This is due to their unusually low detection threshold. We can pick up even just a few picograms per liter of air, which is a thousandth the concentration needed for us to detect “average” odors.
“The sulfanyl alcohols are secreted by sweat glands as odorless precursors containing the amino acid cysteine,” Natsch says. “Bacterial enzymes that can break carbon-sulfur bonds release the sulfanyl alcohols from these conjugates.” His team isolated the gene that codes for the enzyme cystathionine-b-lyase from corynebacteria and transferred it to Escherichia coli. Enzymes extracted from the modified microbe transformed samples of odorless armpit sweat into the typically pungent underarm scent within minutes.
“Now that we know the enzyme (in addition to the enzyme we isolated previously), we can screen for inhibitors of the enzyme,” Natsch told Reactive Reports. “Specific inhibitors of these different axillary odor-releasing enzymes may be useful in the future to manufacture more specific deodorants, which specifically target the odor formation.” Current deodorants simply kill the entire skin microflora while antiperspirants just clog up the sweat glands.
Axillary odor might one day be inhibited rather than masked (Photo by David Bradley)
The enzymes might also be used in the biotechnological production of flavor components. “This has been patented before and we are not following this route,” Natsch told us.
It is not yet known why we are so sensitive to thiols compared to their alcohol equivalents. In the fragrance industry, however, thiols to produce “cassis” notes and specific citrus fruit odors are used at 1000 times lower concentrations than other fragrance components. The 2004 Nobel Prize in Medicine or Physiology rewarded studies on olfactory receptor identification, so adds Natsch, it is likely that molecular olfaction will receive more scientific attention in coming years and perhaps the thiol-binding receptors will be identified. Either way, it is a smelly business.
Chem. Biodiv. 2004, 1, 1022-1035, 1058-1072; http://www.chembiodiv.ch